LTC3802_15 Datasheet, PDF (15/28 Pages) Linear Technology – Dual 550kHz Synchronous 2-Phase DC/DC Controller with Programmable Up/Down Tracking

LTC3802
APPLICATIO S I FOR ATIO
The current limit detector is connected to an internal
100µA current source. Once current limit occurs, this
current begins to discharge the soft-start capacitor at
RUN/SS, reducing the duty cycle and controlling the
output voltage until the current drops below the limit. The
soft-start capacitor needs to move a fair amount before it
has any effect on the duty cycle, adding a delay until the
current limit takes effect. This allows the LTC3802 to
experience brief overload conditions without affecting the
output voltage regulation. The delay also acts as a pole in
the current limit loop to enhance loop stability.
Under severe short-circuit conditions, if the load current is
1.5 times larger than the programmed current limit thresh-
old, the LTC3802 shuts off the top MOSFET immediately.
This stops the increase in the inductor current. At this
moment, if CMPIN is 10% lower than its nominal value, the
LTC3802 hard current limit latches and discharges the
RUN/SS capacitor with a current source of more than 1mA
until RUN/SS hits its shutdown threshold. Once RUN/SS
is completely discharged, the LTC3802 cycles its soft-
start again.
Programming the current limit on the LTC3802 is straight-
forward. To set the current limit, calculate the expected
voltage drop across QB at the maximum desired current:
V_PROG = (I_LIMIT)(R_DS(ON))
I_LIMIT should be set much higher than the expected oper-
ating current, to allow for MOSFET R_DS(ON) changes with
temperature. Power MOSFET R_DS(ON) varies from MOSFET
to MOSFET, limiting the accuracy obtainable from the
LTC3802 current limit loop. Setting I_LIMIT to 150% of the
maximum normal operating current is usually safe and will
adequately protect the power components if they are
chosen properly. Note that ringing on the switch node can
cause an error for the current limit threshold. This factor
will change depending on the layout. The SW node should
have minimum routing from the MOSFETs to the LTC3802
to reduce parasitic inductor and hence ringing. V_PROG is
then programmed at the I_MAX pin using the internal 10µA
pull-up current and an external resistor:
R_IMAX
=
5
• V_PROG
10µA
The resulting value of R_IMAX should be checked in an
actual circuit to ensure that the current circuit kicks in as
expected. Circuits that use very low values for R_IMAX
(<25k) should be checked carefully, since small changes
in R_IMAX can cause large I_LIMIT changes when the switch
node ringing makes up a large percentage of the total
V_PROG value. If V_PROG is set too low, the LTC3802 may fail
to start up. The LTC3802 current limit is designed prima-
rily as a disaster preventing, “no blow up” circuit, and is
not useful as a precision current regulator.
The LTC3802 bottom MOSFET V_DS current sensing archi-
tecture not only eliminates the external current sense
resistors and the corresponding power losses in the high
current paths, it allows a wide range of output voltage
setting, including extremely low duty cycle operation. On
the other hand, for high input voltage with small output
inductance applications, care must be taken to avoid
inductor saturation during dead-short conditions. As soon
as the output short circuits, the controller instantaneously
enters maximum duty cycle operation.
During the top MOSFET on interval, the current compara-
tor is not monitoring the current and there is no current
limit action until the bottom MOSFET turns on and the
inductor current exceeds its hard current limit threshold.
Typically, the top MOSFET and the inductor need to
withstand one clock period of transient high current op-
eration until the hard current limit operation engages.
Peak currents can exceed 6 times the maximum DC output
current during this period. Most MOSFETs allow 10µs of
high current and this short duration of current should not
damage the MOSFET. Nevertheless, it is a good idea to
reduce the peak inductor current. This can be achieved by
having a larger inductance to limit the short-circuit current
slew rate, or an inductor with a saturation current that is
higher than the hard current limit threshold. Alternatively,
an inductor core material with a softer saturation charac-
teristic such as iron powder can be used.
Shutdown/Soft-Start
The RUN/SS pin performs two functions: when pulled to
ground it shuts down the LTC3802, and it acts as a
conventional soft-start pin, enforcing a maximum duty
cycle limit proportional to the voltage at RUN/SS. An
3802f
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